Biology subjects

Seel, T.

Publications and source records attributed to Seel, T..

4 recordsLinked to original sources

bioRxiv2025

High-fidelity, but hypometric spatial localization of afterimages across saccades

Humans typically perceive their visual world as stable and continuous, despite frequent shifts of the retinotopic reference frame caused by saccades. This visual stability is paralleled by afterimage movement across saccades: Although retinotopically stable, afterimages appear to move in egocentric space wherever the eye moves. To investigate the mechanisms underlying this phenomenon, we tasked human observers to localize afterimages relative to briefly flashed probes in complete darkness. This psychophysical tracking of afterimages was accompanied by eye tracking, allowing us to fit a dedicated computational model to accurately predict afterimage movement based on the size of eye movements. The gain of afterimage movement was significantly hypometric, remained unaffected by post-saccadic visual feedback and saccadic adaptation, and was inversely related to saccade gain. These findings suggest that afterimage movement is driven by efference-based, feedforward prediction of visual consequences of saccades and demonstrate the potential of the afterimage-tracking technique for studying visual stability. SignificanceIn low-light environments, brief high-intensity visual stimulation can induce long-lasting retinal afterimages. When observers then make eye movements to explore their visual environment, these afterimages - albeit fixed in the retinotopic frame of reference - appear to move in egocentric space wherever the eye moves. Even though this phenomenon has been known for centuries, the underlying computations remained unexplained. Tracking eye and afterimage positions simultaneously, we found that perceived afterimage position was accurately predicted by eye position across a variety of visuomotor conditions, whereby the eye movements size was however systematically underestimated by the visual system. Considering a parsimonious model of visual localization, afterimage movement can be understood as a consequence of feedforward predictions of the visual consequences of impending eye movements.

neuroscience↗

bioRxiv2025

Early visual signatures and benefits of intra-saccadic motion streaks

Eye movements routinely induce motion streaks as they shift visual projections across the retina at high speeds. To investigate the visual consequences of intra-saccadic motion streaks, we co-registered eye tracking and EEG while gaze-contingently shifting target objects during saccades, presenting either continuous, streaky or apparent, step-like motion in four directions. We found significant reductions of secondary saccade latency, as well as improved decoding of the post-saccadic target location from the EEG signal when motion streaks were available. These signals arose as early as 50 ms after saccade offset and had a clear occipital topography. Using a physiologically plausible visual processing model, we provide evidence that the targets motion trajectory is coded in orientation-selective channels and that speed of gaze correction was linked to the visual dynamics arising from the combination of saccadic and target motion, providing a parsimonious explanation of the behavioral benefits of intra-saccadic motion streaks.

neuroscience↗

bioRxiv2023

Comparing sparse inertial sensor setups for sagittal-plane walking and running reconstructions

Estimating spatiotemporal, kinematic, and kinetic movement variables with little obtrusion to the user is critical for clinical and sports applications. Previously, we developed an approach to estimate these variables from measurements with seven lower-body inertial sensors, i.e., the full setup, using optimal control simulations. Here, we investigated if this approach is similarly accurate when using sparse sensor setups with less inertial sensors. To estimate the movement variables, we solved optimal control problems on sagittal plane lower-body musculoskeletal models, in which an objective was optimized that combined tracking of accelerometer and gyroscope data with minimizing muscular effort. We created simulations for 10 participants at three walking and three running speeds, using seven sensor setups with between two and seven sensors located at the feet, shank, thighs, and/or pelvis. We calculated the correlation and root mean square deviations (RMSDs) between the estimated movement variables and those from inverse analysis using optical motion capture (OMC) and force plate data. We found that correlations between IMU- and OMC-based variables were high for all sensor setups, while including all sensors did not necessarily lead to the smallest RMSDs. Setups without a pelvis sensor led to too much forward trunk lean and inaccurate spatiotemporal variables. RMSDs were highest for the setup with two foot-worn IMUs. The smallest setup that estimated joint angles as accurately as the full setup (<1 degree difference in RMSD) was the setup with IMUs at the feet and thighs. The mean correlations for joint angles, moments, and ground reaction forces were at least 0.8 for walking and 0.9 for running when either a pelvic sensor or thigh sensors were included. Therefore, we conclude that we can accurately perform a comprehensive sagittal-plane motion analysis with sparse sensor setups when sensors are placed on the feet and on either the pelvis or the thighs.

bioengineering↗

bioRxiv2023

Saccadic omission revisited: What saccade-induced smear looks like

During active visual exploration, saccadic eye movements rapidly shift the visual image across the human retina. Although these high-speed shifts occur at a high rate and introduce considerable amounts of motion smear during natural vision, our perceptual experience is oblivious to it. This saccadic omission, however, does not entail that saccadeinduced motion smear cannot be perceived in principle. Using tachistoscopic displays of natural scenes, we rendered saccade-induced smear highly conspicuous. By systematically manipulating peri-saccadic display durations we studied the dynamics of smear in a time-resolved manner, assessing identification performance of smeared scenes, as well as perceived smear amount and direction. Both measures showed distinctive, U-shaped time courses throughout the saccade, indicating that generation and reduction of perceived smear occurred during saccades. Moreover, low spatial frequencies and orientations parallel to the direction of the ongoing saccade were identified as the predominant visual features encoded in motion smear. We explain these findings using computational models that assume no more than saccadic velocity and human contrast sensitivity profiles, and present a motion-filter model capable of predicting observers perceived amount of smear based on their eyes trajectories, suggesting a direct link between perceptual and saccade dynamics. Replays of the visual consequences of saccades during fixation led to virtually identical results as actively making saccades, whereas the additional simulation of perisaccadic contrast suppression heavily reduced this similarity, providing strong evidence that no extra-retinal process was needed to explain our results. Saccadic omission of motion smear may be conceptualized as a parsimonious visual mechanism that emerges naturally from the interplay of retinal consequences of saccades and early visual processing.

neuroscience↗